This invention is to provide a curved magnetic latch or holding device specifically in the form of a magnetic cylinder (or a section of a magnetic cylinder) for use in holding printing plates, embossing plates, and die-cutting plates and the like.
U.S. Pat. No. 5,627,505 by Iwaszek describes a magnetic cylinder or drum used for the same purpose as the instant invention in which a series of circumferentially-spaced recesses or grooves are formed transversely or axially across the drum from one side edge to the other with magnetic elements comprising permanent magnets with interspaced pole pieces inserted in the grooves or recesses. In the Iwaszek patent the magnets are arranged with polarity in a circumferential direction.
U.S. Pat. No. 5,938,579 to Cavazos also shows and describes a magnetic cylinder or drum used for a similar purpose. The Cavazos cylinder similarly has circumferentially-spaced slots or recesses extending axially across the drum with magnetic elements comprising permanent magnets and interposed pole pieces inserted in the grooves or slots with the magnets polarized in an axial direction.
In both of the magnetic cylinders described above, the magnetic force armaturing with and holding down the printing plate or embossing or die-cutting plate is concentrated in the area directly over the grooves or slots in which the magnetic elements are resting. In some cases a printing plate or embossing plate or die-cutting plate or the like when placed on the drum or cylinder may have an edge, either a leading edge or a trailing edge or both, resting in the space between the recesses or grooves where there is little if any magnetic holding force to hold the edge down tightly against the cylinder as it is rotating. In some applications, particularly in the field of embossing using magnetic cylinders to hold embossing plates, these dead spots can cause problems. The present invention is aimed at eliminating or at least minimizing the dead spots so that wherever the leading and/or trailing edge of the plate is placed, there will be at least some significant magnetic holding force holding the edge tightly to the cylinder.
A series or set of recesses or pockets which are circumferentially-spaced from one another formed on the outer surface of a cylinder or drum of non-magnetic material contain magnetic elements comprising permanent magnets and interposed pole pieces. Another similar series or set of circumferentially-spaced pockets or recesses also formed on the outer surface of the drum and containing similar magnetic elements are displaced or offset circumferentially with respect to the first-mentioned set so that they align in an axial direction with the spaces between the pockets in the first mentioned set of pockets. Additional sets of pockets with magnetic elements may be formed transversely or across the width or side-to-side edges of the cylinder so that no matter where a leading or trailing edge of a printing plate or embossing plate is placed on the cylinder, it will cross over a magnetic element contained in one or more recesses so that some significant amount of magnetic holding force is applied to hold the edge of the plate firmly in place on the cylinder.